In conventional RF transceivers filtering is used in at least one of the Intermediate Frequency (IF) and Base Band (BB) sections. However, the filtering has been found to induce distortion into the modulated signal due to non-ideal amplitude and phase characteristics of the filters.
The trend is to increase the RF bandwidth in modem RF communication systems, such as the so-called third generation (3G) wireless communication systems. For example, in the wide bandwidth code division multiple access (WCDMA) system each one of the 12, 5 MHz wide RF channels uses a significant proportion of the total bandwidth allocated to the WCDMA system. In the U.S. WCDMA system the receive band (RX) extends from 1930 MHz to 1990 MHZ (a total of 12, 5 MHz wide radio channels) while the transmit band (TX) extends from 1850 MHz to 1910 MHz (also 12, 5 MHz radio channels). There is a 20 MHz spacing between the RX and TX frequency bands (i.e., the frequency range between 1910 MHz and 1930 MHz.)
A problem exists in the lowest RX channel (1930 MHz to 1935 MHz), since the upper end of the TX band (1910 MHz) is only 20 MHz away from lowest-most RX band edge. In order to obtain a high TX band attenuation the antenna-to-RX branch response of the antenna duplexer is required to be quite steep in the lowest RX channel, and the same situation exists for the highest TX channel.
FIGS. 1A and 1B illustrate this problem, and assume the use of 12, 5 MHz wide radio channels in the 1930 MHz to 1990 MHZ and 1850 MHz to 1910 MHz radio frequency bands. It can be seen that the amplitude and phase response of channels 1 and 12 (CH1 and CH12) are very different as compared to the amplitude and phase response of channels 2-11. This is true for both the RF receiver (FIG. 1A) and the RF transmitter (FIG. 1B). If the amplitude response and/or group delay slope in CH1 and CH12 is sufficiently large it can cause distortion in the received signal that is manifested as an increase in the BER (Bit Error Rate). Similarly, on the transmitter side the slope in the amplitude response and/or group delay can cause distortion in the transmitted signal that can be seen in an increase in the EVM (Error Vector Magnitude). It can be shown that the highest TX channel and the lowest RX channel experience the greatest problems resulting from RF band filter distortion.
The implication is that the amplitude and phase distortion induced by the RF band filters becomes an important consideration. This problem is made particularly acute in the WCDMA system for the United States, where the duplex separation between the receive and transmit bands is only 20 MHz, as compared to 130 MHz for the European WCDMA system. Furthermore, both the antenna duplexer component and the RF filters have very similar frequency responses, which compounds the filter-induced amplitude and phase distortion problems. If the RF distortion due to the RF duplexer and/or the RF filters becomes too severe then either one or both of the BER (in the receive side) and the error vector magnitude EVM (in the transmit side)of the RF transceiver is degraded.
In order to obtain high data rate operation a high signal-to-noise ratio (SNR) is required. However, the degradation in the BER due to the RF filter ripple limits the SNR and therefore limits as well the maximum data rates that can be achieved and sustained.
A need thus exists to address and to solve this problem.